The present invention relates to snapshots of file systems in data storage systems.
Files exist to store information on storage devices (e.g., magnetic disks) and allow the information to be retrieved later. A file system is a collection of files and directories plus operations on them. To keep track of files, file systems have directories. A directory entry provides the information needed to find the blocks associated with a given file. Many file systems today are organized in a general hierarchy (i.e., a tree of directories) because it gives users the ability to organize their files by creating subdirectories. Each file may be specified by giving the absolute path name from the root directory to the file. Every file system contains file attributes such as each file owner and creation time and must be stored somewhere such as in a directory entry.
A snapshot of a file system will capture the content (i.e., files and directories) at an instant in time. A snapshot results in two data images: (1) the active data that an application can read and write as soon as the snapshot is created and (2) the snapshot data. Snapshots can be taken periodically, hourly, daily, or weekly or on user demand. They are useful for a variety of applications including recovery of earlier versions of a file following an unintended deletion or modification, backup, data mining, or testing of software.
The need for high data availability often requires frequent snapshots that consume resources such as memory, internal memory bandwidth, storage device capacity and the storage device bandwidth. Some important issues for snapshots of file systems is how to manage the allocation of space in the storage devices, how to keep track of the blocks of a given file, and how to make snapshots of file systems work efficiently and reliably.